Fixes a couple bugs...
- Pixel font sizes in css would render as point sizes.
- For em font sizes, where the parent size was set and not default, we would
incorrectly convert a pixel value from points to pixels.
We'll always grab the default font size in pixels so we don't keep confusing
things.
Worth noting that gtk css font-size will still behave differently than the
web. Pango interprets font-size differently.

This is in preparation for calc(), as calc(50% - 5px) is valid CSS
but has 2 units. Instead, add a function to query a value's dimension
(so we can differentiate lengths from numbers) and add a function to
query if the value contains percentages.

Parsing a shorthand background property was running into unexpected
errors when trying position values where there were none. To fix this,
introduce a try_parse variant of the position parse function that
silently returns NULL.